The present invention relates generally to multistand metal rolling mills, and more particularly to a method of improved control of interstand tension in such mills.
Multi-stand cold rolling mills equipped with interstand tensiometers generally employ two forms of interstand tension control, i.e., by stand speed and by roll gap. During threading, tension is commonly controlled by stand speed adjustments. When the strip has entered all stands and the exit reel, the mill is accelerated from threading speed to run speed; the tension control method is transferred from stand speed control to roll gap control, and stand speeds are restored to the values required for the target thickness and desired reduction pattern.
Accurate control of strip thickness is not possible before the transition from tension control by speed to tension control by gap, and it is therefore essential that this occur as early as possible in the rolling process. In practice, the transition to tension control by roll gap usually occurs at or close to thread speed in order to reduce the amount of off gage strip. These control strategies and operating practices are well known and have been thoroughly described in the rolling literature, for example "Thickness Control in Cold Rolling" by D. J. Fapiano and D. E. Steeper, Iron and Steel Engineer, November 1983, and "New Approaches to Cold Mill Gage Control" by W. D. King and R. M. Sills, AISE Yearly Proceedings, 1973, p. 187.
While tension control by roll gap adjustment is widely used and generally effective, its effectiveness depends upon good control of stand speeds. Control of tension by roll gap produces uniform strip thickness if, and only if, stand speed relationships are maintained constant. Under this condition, any change in tension can properly be attributed to a change in strip thickness, which will then be corrected when the roll gap is changed to restore tension to the desired level.
The principal weakness in prior art embodiments of this strategy is that any unplanned change in stand speed produces thickness errors. For example, if during acceleration, imperfect speed control causes the upstream stand speed to fall relative to the downstream stand speed, strip tension between the stands would increase. If no tension control action occurred, strip thickness leaving the downstream stand would decrease slightly because of the entry tension increase. With tension control by gap, the tension regulator would close the downstream stand roll gap to restore tension to the reference level, thereby further reducing strip thickness. This vulnerability to speed control errors has focused attention on needed improvements in main drive speed control in new or recently modernized mills. However, a large number of existing mills employ drive and control equipment of older design whose speed accuracy cannot be improved at an acceptable cost.
Field experience has shown that speed ratio errors of 3% to 5% are common during low speed accelerations through the roll stands. When tension control is by roll gap, such errors result in strip thickness errors of a corresponding amount. Off-gage strip results in loss of saleable strip. The problem is most severe at low speed, since load disturbances of a given magnitude produce larger per-unit speed disturbances at lower speeds. The interstand tension change is proportional to the change in interstand strain which results from changes in the ratio of adjacent stand speeds. Thus, it is the change in per-unit speed, rather than absolute speed, which determines the resulting tension change.
An alternative strategy would be to continue tension control by speed until some higher speed has been reached. Such a strategy, however, sacrifices control of gage variations resulting from changes in material deformation resistance or from changes in the roll gap caused by roll thermal expansion or backup roll eccentricity.
An ideal tension control method would be one which retains the advantages of tension control by roll gap but reduces or eliminates the sensitivity to speed errors exhibited by previous embodiments of that strategy.
It is, therefore, an objective of the present invention to provide an improved method of rolling metal strip.
It is a further objective to provide a method of strip tension control which reduces strip thickness errors due to unplanned variations in rolling mill speeds.